A balun is a device that joins a balanced line (one that has two conductors, with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that has just one conductor and a ground, such as a coaxial cable). A balun is a type of transformer: it is used to convert an unbalanced signal to a balanced one or vice versa. Baluns isolate a transmission line and provide a balanced output. A typical use for a balun is in a television antenna. The term is derived by combining balanced and unbalanced.
Integrating components, like baluns, becomes a real requirement if not a vital need in Radio-Frequency circuits. Balun components are frequently used in radio-frequency circuits, mainly to ensure impedance transformation, and signals differential to single-ended conversion, or vice-versa. FIGS. 1 and 2 are given to show for instance block diagrams with balun use, respectively through a receiver and a transmitter path.
In a balun, one pair of terminals is balanced, that is, the currents are equal in magnitude and opposite in phase. The other pair of terminals is unbalanced; one side is connected to electrical ground and the other carries the signal. Balun transformers can be used between various parts of a wireless or cable communications system.
In FIG. 1, a balun is used after a Low Noise Amplifier (LNA) to convert single-ended received signals to differential at a mixer's input.
In FIG. 2, a balun is used after the last amplification stage, to convert output differential signals to single-ended before being connected to an antenna throughout an output matching network.
In the receiver case some problems occur. The balun, being naturally placed before the mixer and almost at the beginning of the receiver chain, presents a high sensitivity level to coupled magnetic fields and interferers, which could seriously impact the global receiver performances. The need of a low-sensitivity balun to radiated magnetic fields and interferers becomes crucial.
Furthermore, the output differential impedance presented to the mixer has to prevent a mismatch that will impact the even-harmonic rejection of the mixer and may generate even harmonic distortion.
Also, attention has to be paid to the balun's insertion loss, since a high insertion loss will directly impact the input noise figure.
In the transmitter case some problems occur. The balun, carrying high power signals, becomes then a very high noise radiation source. A need for low magnetic field radiation structure is present.
Furthermore, it should be prevented that common mode signals and noise will be converted to the single-ended output.
Also low insertion loss is required to save power.
Thus, such components are commonly used in communications devices to achieve impedance transformation and single-ended to differential conversion or vice-versa. However, many prior art components do not ensure low magnetic field radiation and low sensitivity to coupled interferers. Further, components are not provided wherein a mutual coupling helps to realize a balun in a compact space area. Proposed structures no not correct for many other drawbacks and issues, as the ones mentioned above. Typically the prior art baluns further considerably increases the insertion loss characteristics, and suffers form common mode injected noise.
Further, it is important for a balun to have low magnetic field radiation and low sensitivity to coupled interferers.
U.S. Pat. No. 5,477,204 describes a balun as shown in FIG. 3. This disclosure is regarded as closest prior art.
In fact, in order to match the layout and the electrical schematic in the above disclosure (FIGS. 3A & 3B), the 107 connection has to be shifted to the bottom side—where the dashed rectangle is added—so it becomes connected in series to 104, as FIG. 3B says. Actually, as it's drawn on FIG. 3A, the 107 is connected in series to 112, instead of 104, and 112 is the single-ended input of the balun, which does not seem to make sense.
The main problem of this structure lies in the coupling and magnetic transfer between the primary and secondary loops. The length of wires comprising the primary (116-120) and secondary (118-122) are well equal, as the patent mentions it. However, and due to its structure, the primary (routed in white) is longer and has more turns than the secondary (routed in dashed) on the left side, and shorter with less turns on the right side. This leads to the following drawbacks.
Further, the actual geometrical middle points of the primary and secondary are not located closely to each other. As it is, this will result in a transfer from the primary to the secondary loops.
Since the primary has more turns than the secondary on the left side and less on the right, the consequence is that the generated current by induction in the secondary will be higher in the left loop (i1) than in the right loop (i2) (see FIG. 3C). This is a serious drawback.
Further, the output current (104, 110), and output impedance are not any longer fully differential, and a mismatch is created.
A next drawback is that a part of the induced current (i1-i2), will flow to the ground via 107, and as such it will degrade the insertion loss, since this current is regarded as a wasted power.
It is noted that the crossed coupling coefficients do not change anything to these conclusions, since they will add equal contributions. Therefore, the disadvantages mentioned above remain existent.
Now if we look at this balun, as a differential to single-ended transformer, we 25 will find the same disadvantages, namely: the input impedance not being fully differential; and there is degraded insertion loss due to the current conduction through 107.
Furthermore, another drawback can be mentioned in this case, which is the bad rejection of any common mode noise or signal injected at the balanced side input (104, 110), which will result in an undesirable output current on the unbalanced side (112, 114), due to the couplings between the primary and secondary loops.
Thus, the disclosed balun in U.S. Pat. No. 5,477,204 still has many disadvantages. Several other baluns are known from the prior art.
U.S. Pat. No. 6,683,510 A1 (D1) discloses a coupled transmission line balun construction that employs two pairs of planar interleaved spiral coils formed on an electrically insulating or semi-insulating substrate defining a planar structure. One coil in each pair is connected in series to define the input transmission line of the balun, with one end of that transmission line being open circuit The balun provides an ultra-wide bandwidth characteristic in the frequencies of interest for MMIC devices, is fabricated using the same techniques employed with fabrication of MMIC devices, and is of a physical size that lends itself to application within MMIC devices. The balun is symmetrical about its centre line. The first and the second loops have the same length and number of turns in the right and left side.
However, U.S. Pat. No. 6,683,510 A1 does not describe that the balun is eight-shaped and the geometrical and electrical middle point are superposed and at the same level. These features are not known.
U.S. Pat. No. 7,199,682 B1 (D2) discloses a mode-switching transformer having a first conductive line formed of two primary windings electrically in series, and the length of which is a function of a central operating frequency of the transformer; and a second conductive line formed of two secondary windings electrically in series, coupled two-by-two with the primary windings, the length of which is a function of said central frequency. The conductive lines exhibit different widths selected according to the desired impedance ratio of the transformer.
US 2006/097820 A1 (D3) discloses a printed circuit board built-in type planar balun which can be easily incorporated in a printed circuit board without increasing the number of layers and lowering the functions thereof is provided. A balanced signal transmission line 1 and an unbalanced signal transmission line 2 are formed on a same plane, with the sides being opposed to each other. Dielectric layers 3 are provided between these transmission lines, and between the transmission line and a ground potential layer 4, which is arranged substantially parallel to the lines 1 and 2 and spaced at a predetermined distance.
U.S. Pat. No. 6,653,910 B1 (D4) discloses a monolithically integrable spiral balun comprising a substrate having first, second, third, and fourth transmission lines formed thereon. The first transmission line has a first end coupled to receive an input signal and has a second end. The first transmission line forms a spiral that winds in a first direction from its first end to its second end. The second transmission line has a first end and has a second end electrically coupled to the second end of the first transmission line. The second transmission line forms a second spiral that winds in a second direction from its first end to its second end. The third transmission line has a first end for providing a first output and a second end for coupling to a first potential. The third transmission line forms a third spiral that interleaves the first spiral and winds in the second direction from its first end to its second end. A fourth transmission line has a first end for providing a second output and a second end for coupling to a second potential. The fourth transmission line forms a fourth spiral that interleaves the second transmission line and winds in the first direction from its first end to its second end.
EP 1424770 B1 (D5) discloses a wide band mixer is provided that mitigates local oscillator leakage. A LO signal is provided to a 180 degree splitter that provides a first intermediate LO signal and a second intermediate LO signal 180 DEG out of phase from one another. Since both the first and second intermediate LO signals are 180 DEG out of phase, the fundamental LO leakage is mitigated at the RF port by the in-phase combination of the 180 DEG out of phase LO tones canceling one another out and providing strong LO/RF rejection. An RF or microwave input signal is provided to a power splitter to provide a first intermediate RF signal and a second intermediate RF signal. The first intermediate LO signal is mixed with the first intermediate RF signal and the second intermediate LO signal is mixed with the second intermediate RF signal to provide an intermediate frequency signal at the output of the mixer.
U.S. Pat. No. 5,818,308 A1 (D6) discloses a coupled line element that has a plurality of coupled lines equalized to each other in line length to set a phase difference between output signals within a desired range. The coupled lines are wound to form a spiral section. An extension is formed continuously with an end of one or more of the coupled lines fanning an inner winding in the spiral section. The line length of this coupled line is thereby equalized to that of the coupled line forming an outermost winding in the spiral section. Alternatively, the coupled lines can be formed into two spiral sections that are wound in opposite directions.
US 2006/087383 A1 (D7) discloses a balun including a pair of metal coil structures and an intervening dielectric layer having a thickness that is selected in response an operating frequency of the balun. The thickness of the dielectric layer may be used to tune the balun and enhance its self-inductance at its operating frequency. In addition, a balun with a pair of metal coil structures formed with an asymmetry that is selected to minimize an amplitude error in its output signal. A balun according the present teachings may also include an asymmetry in the positioning of its output terminals. The positioning of the output terminals of a balun may be adjusted to minimize phase errors at its output signal.
In summary, D2-D5 describe symmetrical baluns. D6 mentions that the sides are symmetrical, but the figure shows asymmetry in terms of length of the loops. D7 discloses an asymmetric balun.
Further, D1, D3-D5 and D7 disclose a so-called “anti-8” structure, and further D5 proposes in FIGS. 5 and 6 a combiner/splitter design rather than a balun.
An anti-8-shaped structure has its currents flowing in the same directions on right and left loops. This creates external magnetic far-fields in the same directions, which will be summed, and increases therefore the radiation and coupling characteristics of such a device. The mutual coupling will also decrease since internal fields are in opposite directions inside each loop. Thus, the “Anti-8-shape” structure presents globally less performance, regarding mainly coupling and inductance value.
Also, U.S. Pat. Nos. 6,097,273 and 6,396,362 disclose baluns, which are based on the principle of an anti-eight shaped structure. The currents flow in such a way that, instead of increasing the internal field and decreasing the external one, the resulting magnetic fields tend to lessen each other inside the turns and enhance the outside field, and that has a direct degrading consequence on the insertion loss levels and magnetic transfer efficiency.
A problem with the above disclosures is that no baluns are provided that have low magnetic field radiation and low sensitivity to coupled interferers.
Thus, prior art baluns suffer from one or more of the above problems or drawbacks. Thus, there is still a need to solve one or more of the above problems and/or overcome one or more of the drawbacks, by providing further optimized baluns.
Thus, the present invention aims to solve one or more of the above-mentioned problems, such as with insertion loss.